Plastic materials have been widely used in various optical applications, particularly in manufacturing spectacle lenses, because of their light weight, toughness and easiness of dyeing. Optical materials, particularly spectacle lenses are required to have, in addition to a low specific gravity, a high clearness, a low yellowness, optical properties such as a high refractive index and a large Abbe's number. Since a large refractive index reduces the thickness of a lens, many examination has been made thereon to date and many novel compounds have been proposed. Recently, many organic compounds having sulfur atom and/or selenium atom have been proposed to achieve a high refractive index and a high Abbe's number. For example, Japanese Patent Application Laid-Open No. 5-148340 proposes a branched polythiol compounds having four sulfur atoms, Japanese Patent Application Laid-Open No. 6-192250 proposes a dithiol compound having a dithiane ring structure, Japanese Patent Application Laid-Open No. 11-130771 proposes a cyclic compound having selenium atom, Japanese Patent Application Laid-Open No. 11-124363 proposes a polythiol compound having selenium atom, Japanese Patent Application Laid-Open No. 9-110979 proposes a straight-chain polyepisulfide compound, Japanese Patent Application Laid-Open No. 9-71580 proposes a branched polyepisulfide compound, Japanese Patent Application Laid-Open No. 9-255781 proposes a polyepisulfide compound having a cyclic structure, and Japanese Patent Application Laid-Open No. 11-140046 proposes a polyepisulfide compound having selenium atom. However, in the proposed techniques, the refractive index is determined mainly by the chemical structure of the organic compounds having sulfur atom and/or selenium atom. Therefore, it has been demanded to develop a compound with a higher sulfur content to achieve a higher refractive index.